Alkylation is a reaction in which an alkyl group is added to an organic molecule. For example, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. In petroleum refining, the process reacts a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst to produce an upgraded product stream referred to as alkylate. This alkylate is a valuable blending component in the manufacture of gasoline due not only to its high octane rating but because it is free of aromatic components.
Industrial alkylation processes have historically used concentrated hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive. However, it is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry uses anhydrous hydrogen fluoride primarily as a liquid catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline. Years of experience in its manufacture and use have shown that HF can be handled safely, provided the hazards are recognized and precautions taken. Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape creating a vapor cloud that can be spread for some distance. Previous workers in this field approached this problem from the standpoint of containing or neutralizing the HF cloud after its release.
U.S. Pat. Nos. 4,938,935 and 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan teach various methods for containing and/or neutralizing HF acid clouds following accidental releases.
In addition to these efforts directed at making the HF acid circulating in a plant safer by reducing its cloud forming tendencies, there is concern about making every part of the process safer. One significant hazard, which has previously been overlooked by others, is adding fresh HF acid to the plant. A similar problem is re-inventory, that is, the return of the HF acid inventory to the plant after a plant shutdown to permit repair or inspection of equipment.
Now this is done by rotating equipment, typically pumps with seals. Such equipment can and does leak or fail. Standard practice is to use tandem seals which should prevent a catastrophic leak, however failures do happen and leaks can occur.
Usually the fresh acid, added to replenish acid consumed or lost during processing, is relatively pure acid. Fresh acid does not contain sulfolane or other agents which are present in the HF acid circulating in acid inventory of the plant, so it represents a potential threat to the environment even when sulfolane or the like is present in the acid inventory.
Some efforts have been made to reduce the amount of rotating pumps used in HF alkylation units, which are reviewed briefly hereafter.